At this time, the number of aircraft that exceed their originally designed lifetime service objectives is approximately 39% of the modem air transport fleet. Maintenance of these "aged" aircraft is an important issue; especially the evaluation of accumulated structural fatigue that may cause fatigue failure of the aircraft components during flight. It is estimated that about 3% of all aircraft accidents are a result of accumulated structural fatigue .
Cyclical loading caused by pressurization/depressurization as the aircraft climbs and descends is a material contributor to fuselage fatigue and failure. These pressurization cycles account for from 90% to 100% of the fuselage fatigue life. Recent accidents have occurred that have been directly related to fuselage fatigue failure. In these instances, the aircraft's accumulated flight cycles exceeded the original design service objective, resulting in widespread materials fatigue, or skin cracking, around rivet holes. Ultimately structural failure, such as the separation of parts of the fuselage skin from the aircraft, occurred. As a direct result of these accidents, the aviation industry is increasingly concerned about aircraft component failures due to accumulated structural fatigue.
Aircraft also experience wing fatigue failures that are due to the loads placed on the aircraft by turbulence and maneuvers, in addition to the normal wing loads that occur during takeoffs and landings. The individual contributions of each phenomenon depend on the particular route of the individual aircraft, i.e., number of landings per day, turbulence encounters, flight path, and the like. An accepted estimate of the relative contribution to wing fatigue failure of the various components is: Turbulence 35% to 60%; Takeoffs and Landings: 35% to 50%; and, Maneuvers: 10% to 15%.
Presently, airlines routinely inspect aircraft for fatigue damage, typically during a "C" checks, which occur every 2,400 flight-hours. During the "C" check maintenance technicians use non-destructive equipment such as high-frequency eddy current ultrasound and x-rays to detect structural fatigue. However, "C" checks are expensive and cannot reasonably be done on a frequent basis. Thus, there is a need for a method that helps determine when an aircraft should be scheduled for additional fatigue testing, even before the scheduled "C" check.
By knowing the history of a given aircraft in terms of the parameters that affect fatigue life, airlines could adjust their fatigue maintenance schedules to accurately reflect the number of structural load cycles that the aircraft has encountered and thereby permitting safer aircraft operations while possibly reducing unnecessary maintenance actions.